unwritten

A rule, provision, or guideline that is generally accepted but not formally established or enforced. It's become something of an unwritten law that people from different departments only sit together during lunch. It's just an unwritten law that a new president will make his previous tax returns public—seems kind of suspicious if they don't. The unwritten law for many years was that successful athletes were given free passes for bad behavior or poor grades, so long as they remained successful in their sport.

A law, provision, or guideline that is generally accepted but not formally established or enforced. It's become something of an unwritten rule that people from different departments only sit together during lunch. It's just an unwritten rule that a new president will make his previous tax returns public—seems kind of suspicious if they don't. The unwritten rule for many years was that successful athletes were given free passes for bad behavior or poor grades, so long as they remained successful in their sport.

An accepted although informal rule of behavior, as in It's an unwritten law that you lock the gate when you leave the swimming pool. [Mid-1400s]

Rules accepted by custom or tradition rather than codification in a formal body of law. The idea was already expressed by Plato: “There is a written and an unwritten law. Written law is that under which we live in different cities, but that which has arisen from custom is called unwritten law” (quoted by Diogenes Laertius; in Latin, lex no scripta). In a famous legal case in which he succeeded in having his client, Harry Thaw, who was accused of murdering Stanford White, declared insane, Delphin Michael Delmas coined the phrase (1907), “Dementia Americana; the unwritten law.”